Adhesives which are solutions of rubber compounds in organic solvents having a boiling point of at most about 150.degree. C. are used presently in adhesion processes of adhering unvulcanized rubber compounds, or an unvulcanized rubber compound and a vulcanized rubber compound together.
Recently, reductions in organic solvent emissions from adhesives have been investigated from the standpoint of industrial hygiene, prevention of disasters and conservation of oil resources. Attempts have been made not to use organic solvents, to decrease the content of organic solvents by using highly concentrated adhesives and to decrease the area of application of adhesives to the surfaces of rubber compounds by improving the formulation of rubber compounds. However, these attempts have been unsatisfactory in achieving low levels of organic solvents emission without adhesion difficulties. Therefore, it is of urgent necessity to achieve a new adhesive having good adhesion strength and with a low organic solvent content.
Oligomer type adhesives do not provide sufficient adhesiveness and good adhesion after vulcanization.
Latex type adhesives and oil-in-water type adhesives are water-based adhesives, and have such poor wettability to rubber compounds that sufficient tackiness cannot be obtained. Moreover, it is very difficult to remove water of the adhesives from the surface of the rubber compound and the adhesion after vulcanization is deteriorated due to the water present.
It is known that water-in-oil type adhesives have low organic solvent content and good wettability to rubber compounds and dispersed water particles of the adhesives so easily cohere due to mechanical stimuli such as supersonic waves that it is easy to remove water of the adhesives from the surface of rubber compounds.
European Patent Application No. 83305211.1 (corresponding to U.S. patent application Ser. No. 415,496 filed Sept. 7, 1982, now abandoned) discloses a tire tread adhesive comprising a water-in-oil emulsion with an organic solvent solution of a vulcanizable rubber compound as its oil phase. However, it is difficult to manufacture a stable water-in-oil emulsion because of the necessity to disperse water particles measuring several microns in diameter into the oil phase containing a rubber compound comprising a rubber and carbon black.